Electrolyte capacitors such as aluminum and tantalum capacitors with a liquid electrolyte were already replaced with those having a solid electrolyte quite some time ago. In the case of tantalum capacitors, in particular, manganese dioxide is used as the solid electrolyte. For this purpose, manganese nitrate is brought into the porous surface of the metal anode and pyrolyzed, using a complicated, multi-stage process. In addition to the technological effort involved, another disadvantage is that during the pyrolysis or sintering process, aggressive nitrogen dioxide (NO.sub.2) is released, which can result in damage to the metal oxide serving as the dielectric. This is counteracted in that the oxide layer which occurs as a result of forming is made thicker than necessary. However, as a consequence capacity losses have to be accepted.
Electrolyte capacitors which contain electrically conductive organic complex salts on the basis of 7,7,8,8,-tetracyano-1,4-quinodimethane (TCNQ) as the solid electrolyte are also already known (see, for example, U.S. Pat. No. 4,580,855). However, a disadvantage of these TCNQ complexes, which are applied to the formed, i.e. oxidized metal surface in the molten state, is that they can only be worked or melted at temperatures at which their stability limit is already reached, and that over time--especially when overheated--they split off hydrocyanic acid and therefore have a toxic and corrosive effect (see EP-OS 0 340 512).
It is also known to use conductive polymers as the solid electrolyte in electrolyte capacitors (see, for example, EP-OS 0 135 223, 0 264 786 and 0 340 512). Such solid electrolyte capacitors have the advantage, as compared with conventional capacitors with liquid electrolyte, that the electrolyte cannot run out or evaporate. Furthermore, the power loss, i.e. the ESR ("electrolytic serial resistance") is less. In comparison with other solid electrolyte capacitors, better frequency behavior and better capacity utilization can be expected, in addition to advantages from a technological point of view.
The major problem in the implementation of an electrolyte capacitor with a conductive polymer as the solid electrolyte is the efficient placement of the polymer into the anode, which generally has a highly porous surface structure. This is because etched films or sintered compacts produced with extremely fine powder achieve high volume capacities.
Conductive polymers are usually produced by means of electrochemical polymerization (of corresponding monomers), i.e. deposited onto a substrate. Since this deposition presupposes an electron-conductive substrate, this method of procedure cannot be easily transferred to the production of solid electrolytes for electrolyte capacitors. This is because the formed metal anodes of electrolyte capacitors demonstrate dielectric properties.
Methods in which electrochemical polymerization on an aluminum substrate takes place first, and the dielectric barrier layer is only produced afterwards, by application of an external voltage, are known (see EP-OS 0 283 239 and 0 285 728 in this regard). However, a drawback is that the polymer properties are adversely influenced by the subsequent forming process; in addition, the homogeneous formation of the dielectric is disrupted by the polymer.
In other known methods, the problems mentioned are circumvented by the fact that chemical polymerization processes are used. According to EP-OS 0 340 512, the use of polythiophenes with a specific structure as the solid electrolyte in electrolyte capacitors is known. The polythiophenes are produced on metal foils covered with an oxide layer on one side, which are used as anodes, by applying monomeric thiophenes and oxidation agents, preferably in the form of solutions, either separately one after the other, or preferably together, onto the side of the metal foils covered with the oxide layer; the oxidative polymerization is completed, if necessary, by heating the coating. If the thiophene monomer and the oxidation agent are applied separately, the metal foils are preferably first coated with a solution of the oxidation agent and subsequently with the thiophene solution. The solvents are removed by evaporation at room temperature, after the solutions have been applied.
Coating of the anodes can also take place by means of gas phase polymerization (see in this regard: JP-OS 63-314823 and/or "Chemical Abstracts," Vol. 111 (1989), No. 16, 145285s, and JP-OS 01-012514 and/or "Chemical Abstracts," Vol. 110 (1989), No. 26, 241310w). For this, a formed aluminum anode, for example, is first treated with a solution of an oxidation agent, and then exposed to a monomer, such as pyrrole.
A polypyrrole solid electrolyte can also be produced in such a manner (see in this regard: JP-OS 01-049211 and/or "Chemical Abstracts," Vol. 111 (1989), No. 6., 49055w) that the anode is first impregnated with a solution of the monomer and then treated with a solution of an oxidation agent.
The methods of the type mentioned above, in which polymerization takes place by the chemical route, demonstrate a number of disadvantages. The following points are particularly significant:
1. The reduced form of the oxidation agent remains in the polymer layer. This reduces the conductivity of the layer, and furthermore, the stability of the polymer can be adversely influenced and the dielectric can be damaged.
2. Due to the volume loss during the evaporation of the solvent, i.e. due to the fact that deep, fine pores are not reached, filling of the pores is not efficient.
3. The required solutions can generally be processed only over a very limited period of time, i.e. for approximately 1 to 2 h (see EP-OS 0 340 512 in particular, in this regard).